1. Field of Invention
The present invention relates generally to stencil cleaning methods and apparatus, and more particularly to a stencil printer including a stencil wiper assembly having a self-contained vacuum module.
2. Discussion of Related Art
In a typical surface-mount circuit board manufacturing operation, a stencil printer is used to print solder paste onto a circuit board having a pattern of pads or some other conductive surface onto which solder paste will be deposited. The circuit board is automatically fed into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, is used to properly align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. Once a circuit board has been properly aligned with the stencil in the printer, the circuit board is raised to the stencil, solder paste is dispensed onto the stencil, and a wiper blade or squeegee traverses the stencil to force the solder paste through apertures formed in the stencil and onto the board. As the wiper blade is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste is typically dispensed onto the stencil from a standard cartridge.
In some prior art stencil printers, any excess solder paste remaining under the wiper blade after it has fully traversed the stencil remains on the stencil when the wiper blade is returned to its initial position for printing on a second circuit board. Usually, as the wiper blade passes the solder paste over the apertures, minute amounts of solder paste seep through the apertures to accumulate at the bottom side of the stencil. This presents various problems such as the solder paste being inadvertently disposed on the unintended areas of the circuit boards thereby jeopardizing the reliability of the printing process. Also, as the solder paste hardens, it complicates the alignment procedure of a circuit board with the stencil. Therefore, it is highly desirable to remove the excess solder paste that forms on the bottom of the stencil.
U.S. Pat. No. 5,918,544 to Doyle represents one prior art stencil printer having a well-known method and apparatus for cleaning the bottom of the stencil. Doyle discloses a wiping system that is positioned near the vicinity of the stencil and moves beneath the stencil from one end of the stencil to the other end. As the stencil wiper system moves beneath the stencil, it wipes off excess solder paste at the bottom of the stencil.
Specifically, the stencil wiper system includes a paper supply roller containing a roll of web material, such as paper, a take-up roller, at least one guide roller, a hollow solvent tube with numerous small openings formed along the length of the tube, and a vacuum plenum for removing excess moisture and hardened solder paste from the paper as it travels underneath the stencil. During a cleaning operation, a paper winder motor or drive rotates the paper supply roller by driving the take-up roller. The hollow solvent tube is filled with solvent by a solvent pump, which causes the solvent tube to squirt solvent through its numerous holes onto the paper as it passes the solvent tube. The solvent impregnated paper is passed to the vacuum plenum, which holds the paper in place as the stencil moves over the paper, thereby cleaning the stencil.
A disadvantage to the system described in Doyle, as well as other prior art wiper systems, is that a pump mechanism for creating the vacuum (negative pressure) at the vacuum plenum is typically located separate from the stencil wiper system itself. The pump is typically connected to the plenum using several (e.g., four or more) hoses so as to create vacuum at several places along the length of the plenum. Locating the vacuum generator remote from the wiper system has several disadvantages, such as the need to run many hoses through the machinery in order to connect the vacuum generator to the wiper system, which may be clumsy and may increase possibilities of failure of the system. In addition, each hose needs to be protected to avoid any bending or compression that could disrupt the airflow, thus the hoses are typically installed in metal casings in areas where crimping may occur to protect them. This results in a large amount of space being needed to run the several hoses to the vacuum plenum assembly and increased costs associated with the hoses and the protective casings.